Conventional wire-dot impact printers include those equipped with a capacitance detector which detects a displacement of each armature by a sensor electrode disposed opposite the armature.
Such wire-dot impact printers are disclosed, for example, U.S. Pat. Nos. 4,940,343, 5,030,020 and 5,039,238 commonly assigned to the present assignee. In the wire-dot print head of the wire-dot impact printer disclosed in one of the U.S. patents, i.e., U.S. Pat. No. 4,940,343, an armature is normally attracted toward a core by a magnetic flux produced by a permanent magnet so that an associated leaf spring is flexed. In use, the magnetic flux is canceled out by a magnetic flux produced by an associated coil, whereby the armature is released to drive a print wire fixed on a free end of the armature. A printed circuit board is provided in opposition to armatures. Sensor electrodes are formed on the printed circuit so that a small space left between the sensor electrodes and their corresponding armatures. A displacement of each armature is therefore observed by a change in capacitance between the armature and its corresponding sensor electrode. Upon printing, the drive time of the wire-dot print head is controlled in accordance with the displacement of the armature.
On the printed circuit board, the sensor electrodes are formed as many as the armatures. Mutually independent signal lines are connected to the sensor electrodes, respectively. Further, a common line is formed around the sensor electrodes on the printed circuit board to prevent occurrence of interference between the adjacent sensors.
The common line formed around the sensor electrodes on the printed circuit board is, however, provided merely to prevent occurrence of interference between the adjacent sensor electrodes and cannot protect the sensor electrodes from other electrical disturbance such as noise. When an insulated area on the printed circuit board is electrostatically charged by frictional contact or the like, the charge of each sensor electrode itself is apparently increased or decreased due to the charge around the sensor electrode. As a result, a change in potential of the sensor electrode is also increased or decreased, thereby making it impossible to achieve accurate detection of the change.